<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>

</body>
<script>
    var obj = {
        a:"a",
        b:'b'
    };

    Object.preventExtensions(obj);

    obj.c="c";
    console.log(obj);
</script>
</html>